Indirectly heated electronic tube



Oct. 5, 1937. P. KAPTEYN INDIRECTLY HEATED ELECTRONIC TUBE Filed oct. ze, 1954 ffy. 2..

Patented Oct. 5, 1937 A UNITED STATES rari-:nr yorrlcla Application October 26, 19,34, Serial No. 750,168

' In Germany October 30, 1933l sclaims. (o1. 25o-27.5.) y

The present invention relates to indirectly heated'cathodes for electron discharge devices.

In the case of indirectly heated cathodes, and more particularly those having a biiilar filament,

the free ends of the lament, and in certain cases the leads themselves, which for example in the case of 'so-called high-*voltage cathodes' are linked up, with an alternating potential amounting to approximately 50 volts cause a detrimental secondary emission, particularly in case that the emissive surface is produced' by meansof the socalled metal vapour process. This secondary, emission, which contributes appreciably to the total emission, results in humming noises, as its potential reveals considerable uctuations (for example of up to Zl0 volts) in relation to the anode during the duration of an A. C. period. The present invention has for its object a method of avoiding the stated interfering elects, and also an electron discharge valve having an lindirectly heated cathode, in which these effects are unable to occur.

The invention comprises an electron discharge tube including an indirectly heated cathode comprising an insulating tube, an equipotential tube surrounding the mid-portion of said insulating tube, said equipotential tube being provided with a highly emissive surface produced by means of the metal vapour process, the ends of said insulating tube projecting from said equipotential tube being at least partly coated with a black good heat-radiating material, a heating vspiral arranged within said insulatingtube, the length of said heating spiral'corresponding to the length 85 of said equipotential tube and two connecting leads for said spiral, each of said leads projecting into one of the openings of said insulating tube, the diameter of each of said leads being so great that no appreciable heating of said leads isp'- 40 duced by the heating current of said spiral.

The invention also comprises an electron discharge tube including an indirectly heated cathode comprising a rod consisting of insulating material, said rod being provided with two bores, two

45 heating filaments, each of said filaments being arranged in one of said bores, one connecting lead, each of the ends of said connecting lead projecting into one of the upper openings of said bores and being connected to one end of each of 50 said laments, two further connecting leads, each of said further leads projecting into one of the lower openings of said bores and being connected to one of the other ends of said filaments, each of said leads having a diameter which is at least 55 iive times as great as the diameter of each of said filaments, a cap surrounding theV upper part of said rod, an equipotential layer surrounding at least Vthe mid-portion of said rod, and a highly emis'sive substance arranged on said equipotential layer; 5

Two embodiments of the invention are shown by way of example inthe appended drawing.

In Fig. 1 there is shown an equipotential cathode, in which the leads'are introduced at either end of 'the insulating' tube and are connected 10 with the heating iilament preferably by welding, while in Fig. 2 there is shown a cathode having a double-wound `larnent wire inthe arrangement according to the invention. n, 15

Fig. 3 shows how the heating spirals are secured at the connecting leads.

In the drawing I is the insulating tube, which may consist for example of any suitable ceramic material, such as porcelain, magnesia or the like, 20 2 the equipotential tube, 3 the highly emissive layer, and 5 the wound filament, which is secured to current leads 6 and, in accordance with the invention, may preferably possess the same length as the highly emissive layer. The leads possess 25 in accordance with the invention a considerably greater cross-section than -the filament. Preferably the cross-section of the leads lis so chosen that they t exactly into the respective bores of the insulating tube. The leads are rmly connect- 30 ed within Vthe insulating tube with the ends of the filament. In this manner it is accomplished that the leads projecting from the cathode are kept cool, and outside of the insulating tube there are no metallic portions traversed by heating ourrent which possess as high `a temperature as is necessary for producing an appreciable emission.

It is naturally also possible and desirable in accordance with the invention to provide suitable additional means having good heat radiating properties for keeping the ends of the cathode cool. For example in the arrangement of Fig. 1 there may be provided at either end of the equipotential surface a coating of a black substance, such as platinum black or a colloidal dispersion of graphite and/or carbon in a suitable solvent, (aquadag).

In Fig. 2 there is illustrated by way of example an embodiment of an equipotential cathode with a bilar lament. The reference characters are the same as those employed in Fig. 1. In this arrangement the current leads, which possess a low temperature, may, conveniently be employed at the same time for holding the equipotential cathode in the press portion 8 of the valve. The 55 be connected with the electrodes outside the ine` .Y

A sulating tube, for example by spot-welding. At'

the reversing point in the upper partof the cathode there is provided likewise for the purpose of accomplishing the desired eiect a thick, curved lead, andthe upper opening of the cathode is closed by suitable means, for example by 4an adapted layer of porcelain or other ceramic sub-: vr

stance. On the other hand, the equipotential` tube 2g mayY beso constructed as to cover said opening. Y i .The diameter of the feeding leads should be in each case `considerably greater than the diameter of the heating iilament, say, at least five times as Ygreat las the same.

The essential advantageV of the, construction according kto the invention consists in complete elimination; ofl thevdisturbances occurring in the caseof indirectly heated high-voltage cathodes, and more particularly cathodes produced according to a metal vapour process, with a considerably more simple method of production, which is simple to perform from the point of view of manufacture and is considerably cheaper as compared with the methods hitherto employed.

It is naturally also possible to make use in addition of the arrangements as are described in my application Ser. No. 677,526 and to screen off inV special fashion the leads to the heating wire and the remaining electrodes against impinging metallic Vapour resulting upon the activation of the cathode, or against each other in the case ofV any interfering electrons which may result.

The arrangement of the invention'is suitable not only for single-system tubes, butalso more particularly for'multi-stage tubes, as in the case of such tubes alsimple, space-saving construction is of peculiar importance, whereat said systems may comprise one or several plates and/or several grids which may be constructed and arranged in a manner known per se.

For accomplishing the desired effect not only the diameter, 4but the material of the leads Valso plays an important part. According to the invention the ratio of the resistances of the leads and of the filament per unit of length should be preferably not less than 1: 10. A narrowly wound spiral iilainent of tungsten wire of 50p. 4and leads consisting for example of nickel of .4 mm. diameter would correspond to the above condition.

I claim:

1. An indirectly heated cathode comprising a rod consisting of insulating material, said rod being provided with two bores, two heating laments, each of said filaments being arranged in one of said bores, one connecting lead, each of the ends of said connecting lead projecting into one of the upper openings of said bores and being connected to one end of each of said filaments, two further connecting leads, each of said further leads projectingl into one of the lower openings of said boresand being connected to one of the other ends of said filaments, each of said leads having a diameter which is'at least ve times as great as the diameter of each of said laments, a cap surrounding the upper Ypart of said rod, an equipotential layer surrounding at least the mid-portion ofsaid-rod, anda highly emissive substance arrangedhon said equipotential layer.

2. An indirectly heated cathode comprising a rodrconsistinrg of insulating material, said rod beingrprovided with two bores, two heating filaments, each of said filaments being arranged in one of said bores, one connecting lead,reach of the ends of Ysaid connecting lead projecting into one of the upper openings of said bore and being connected to one end of each of said filaments, two further connecting leads, each of said further leads projecting into one of the lower openings of, saidV bores and being connected to one of the other-ends of said filaments, each of said leads having a diameter which is at least ve times as great as-the diameter of each of said laments, apap surrounding the upper part of said rod, an eduipotential layer surrounding at least the midportion of said rod, andra highly emissive substance arranged on said equipotential layer, and two coatings of good heat-radiating material,

each of said coatings being arranged at one'end of said insulating tube.

3. An indirectly heated cathode containing a rodof insulating material, said rod provided with two borings, two heating filaments arranged in said borings, the diameter of said borings being at least ve times as great as the diameter of said laments, said laments connected on the upper ends with a lead and connected on both lower ends with two leads, said leads having the same diameter as said borings and projecting into said borings to approximately a fourth part of the Whole length of said borings for preventing emission of saidV heatingfilamentsV out of said borings, an equipotential layer surrounding atleast the mid-portion ofV said insulating rod coated with an emissive substance, a cap surrounding the upper part of said rod. Y

PAUL KAPTEYN. 

